To manufacture a sandwich panel, for example, a conventionally used forming method employs primary forming using extrusion, in combination with secondary forming using blow (or vacuum). According to such a forming method, an extruded molten resin is directly subjected to blow molding (or vacuum forming). This method thus makes it possible to form a sandwich panel without posing a technical problem such as non-uniform heating, which would otherwise be caused by reheating of an already formed resin. Particularly in this forming method, blow molding (or vacuum forming) is performed by causing an extruded molten resin to droop downward as it is, and then clamping the vertically extending resin. Therefore, it is not necessary to support the molten resin until clamping performed in the secondary forming, unlike in the case of for example, extruding a resin laterally. It is thus possible to feed a resin in a non-contact state from an extrusion die. Such a forming technique is disclosed in, for example, Patent Documents 1 to 3.
In each of the forming techniques disclosed in Patent Documents 1 to 3, a molten resin is allowed to pass through between a pair of rollers, before the molten resin extruded downward is formed within a mold. However, the pair of rollers disclosed in Patent Document 1 is merely pressure rollers for pressure-bonding a sheet-shaped resin extruded from an extrusion head to a decorative sheet. In the techniques disclosed in Patent Documents 2 and 3, on the other hand, a gap between the pair of rollers is set smaller than a thickness of a sheet-shaped molten resin. In addition, a temperature of the rollers is set close to that of the sheet-shaped molten resin. Under such conditions, the sheet-shaped resin is allowed to pass through between the pair of rollers to thereby be formed. It is thus possible to provide a mirrored or glossy surface of the sheet. In these conventional forming techniques, however, the molten resin extruded prior to secondary forming is caused to droop downward as it is. As a result, the following technical problems arise.
The first problem is that draw-down or neck-in, which occurs in a molten sheet, makes a thickness of the sheet prior to forming in a mold uneven in an extruding direction or a width direction of the sheet. When the sheet is finally formed by secondary forming using blow or suction (vacuum), the uneven thickness in the extruding direction of the sheet before the secondary forming affects a thickness of the finished sheet after the secondary forming. To cope with this problem, for example, when, during forming of the sheet, an extrusion speed of the sheet is changed to increase according to the draw-down as the forming proceeds, a thickness of the sheet corresponding to a later stage of the forming can be increased toward an upper part of the sheet. Therefore, it seems possible to cope with the thinning of the upper part of the sheet caused by the draw-down. However, the change in the extrusion speed of the sheet causes a change in an extrusion pressure of the sheet. As a result, a swelling in the molten resin extruded from an extrusion slit changes, which rather accelerates the unevenness of the sheet thickness. Note that Patent Document 3 discloses controlling a pulling speed of a sheet or adjusting a rotation speed of a screw of an extruder according to the draw-down of each sheet. However, this does not suppress or eliminate occurrence of the draw-down of each sheet. In this case, assuming that the draw-down occurs in two sheets, the pulling speed of the sheets is controlled or the rotation speed of the screw of the extruder is adjusted, and consequently, both the sheets are merely supplied into a mold almost at the same time.
It is possible to prevent such draw-down or neck-in to some extent by employing, as the resin used for a sheet, the one having a predetermined or smaller MFR value or melt tension value. In this case, however, materials that can be used are limited, which is not practical. Particularly when forming a thin sheet, the larger the MFR value, the better. For these reasons, limiting the MFR value may not solve the problem.
The second problem is that, when a pair of rollers is used to feed a sheet-shaped molten resin downward in order to eliminate the first technical problem, i.e., the unevenness of the sheet thickness before forming, the sheet-shaped resin may rather get caught in the rollers. More specifically, in order to reliably feed the sheet-shaped molten resin by the pair of rollers, it is important to ensure a dynamic frictional force between a surface of each roller and a surface of the sheet. On the other hand, when the pair of rollers sandwiches the sheet-shaped molten resin and feeds it downward, the pair of rollers is heated by the sheet-shaped resin due to thermal conduction. Therefore, in a stationary state, a temperature of outer surfaces of the pair of rollers is close to a temperature of the sheet-shaped resin. In this case, also in the stationary state, the larger the pressing force of the pair of rollers against the sheet-shaped resin, the easier for the sheet-shaped molten resin to closely adhere to the outer surfaces of the pair of rollers. Therefore, the sheet-shaped resin may be wound around the outer surfaces of the rollers along with the rotation thereof, instead of being fed downward. However, when decreasing the dynamic frictional force by reducing the pressing force, there may be sliding contact between the outer surfaces of the pair of rollers and the surfaces of the sheet-shaped resin, which makes it difficult to reliably feed the sheet-shaped resin downward.
In contrast, in a hollow molding method disclosed in Patent Document 4, an extrusion head extrudes a pair of sheet-shaped resins each having a half-arched cross-sectional shape. Each of the two sheet-shaped resins is supported by roller-shaped guiding members so as to widen a gap between the opposed resins, made into a flat shape, and then allowed to droop in the vicinity of one of mating surfaces of split mold blocks. After that, the split mold blocks are clamped while a pressurized fluid is introduced into the sheet-shaped resin, thereby fondling a hollow object. More specifically, the two sheet-shaped resins, which are extruded by the extrusion head to droop downward, are guided by the guiding members in such a manner as to widen the gap between the resins. A guiding resistance at this time prevents the draw-down of a parison in a portion between the extrusion head and the guiding member. In addition, the parison is formed into a uniform flat shape through the guiding members. Patent Document 4 also describes that the guiding members, which correspond to the pair of rollers, may be driven to rotate synchronously. Patent Document 4 further describes that a temperature controller may be provided inside the guiding member to control a surface temperature of the guiding member to be close to a temperature of the sheet-shaped resin.
However, the prevention of draw-down of a parison disclosed in Patent Document 4 is realized by arranging the pair of roller-shaped guiding members in a direction of widening a gap therebetween. It is merely an optional configuration to drive the guiding members to rotate synchronously and provide the temperature controller inside the guiding member so as to control a surface temperature of the guiding member to be close to a temperature of the sheet-shaped resin. In other words, Patent Document 4 discloses neither a specific configuration to drive the guiding members to rotate synchronously, nor a temperature controller configured to control a surface temperature of the guiding member to be lower than a temperature of the sheet-shaped resin. Furthermore, Patent Document 4 does not even suggest, not to mention disclosing, the technical problems such as sliding contact between the guiding member and the sheet-shaped resin and winding of the sheet-shaped resin around the guiding member, which are caused by the fact that the sheet-shaped resin is fed through the guiding members.
The third problem is that, the more the rotation speed of the rollers increases to improve forming efficiency, the more noticeably the sheet is wound around the outer surfaces of the rollers. However, when the rotation speed of the rollers is decreased to prevent the sheet from being wound around the outer surfaces of the rollers, the forming efficiency is lowered.
The documents that describe the related art are listed below.    Patent Document 1: JP 2000-218682 A    Patent Document 2: JP 3-27922 A    Patent Document 3: JP 11-5248 A    Patent Document 4: JP 04-2087 B